Hemodialysis (or more simply, “dialysis”) is a method for removing waste products such as potassium and urea from the blood, such as in the case of renal failure. During hemodialysis, waste products that have accumulated in the blood because of kidney failure are transferred via mass transfer from the blood across a semi permeable dialysis membrane to a balanced salt solution. The efficiency of a hemodialysis procedure depends on the amount of blood brought into contact with the dialysis membrane. A flow of 250 milliliters of blood per minute under a pressure gradient of 100 millimeters of mercury is considered a minimum requirement for adequate dialysis. Over the past several years, flow rates between 350 milliliters per minute and 400 milliliters per minute have become common. These high flow rates and related pressures give rise to unique issues and challenges associated with dialysis catheters, as compared to other types of catheters.
The long hours and the frequency of the dialysis treatment in patients with renal failure require reliable, continued access to the venous system for blood exchange. Long-term venous access mechanisms commonly used for hemodialysis treatment include vascular access ports, dialysis grafts, and hemodialysis catheters. One type of blood treatment catheter that is well known in the art is a dual or triple-lumen hemodialysis catheter. These catheters are designed to provide long-term access to the venous system for dialysis. The dual-lumen catheter typically has an inflow lumen for withdrawing blood to be treated from a blood vessel and an outflow lumen for returning cleansed blood to the vessel. The distal segment of the catheter is typically positioned at the junction of the superior vena cava and right atrium to obtain a blood flow of sufficient volume to accommodate dialysis treatment requirements. This allows blood to be simultaneously withdrawn from one lumen, to flow into the dialysis circuit, and be returned via the other lumen. Triple lumen catheters function in a similar manner but have an additional smaller lumen which may be used for guidewire insertion, administration and withdrawal of fluids such as drugs or blood sampling, and for injection of contrast media required for imaging procedures.
To optimize blood flow rates during dialysis and reduce treatment times, catheters have been designed to maximize the cross-sectional lumen area of the inflow and outflow lumens. It is well known in the art that blood flow rates are negatively impacted if the cross-sectional area of the lumens does not remain essentially consistent and as large as possible throughout the entire length of the catheter from the proximal portion of the catheter to the distal portion of the catheter. Catheters with large, consistent luminal space typically have exit ports with blunt or flat-faced open tips, so as not to compromise the luminal area. Typically the exit port at the distal end of the catheter is cut at a 90-degree angle to the axis of the catheter.
One possible complication of dialysis catheters and other indwelling vascular medical devices is the aggregation of platelets on the surface of these devices, promoting thrombus formation, which may lead to catheter complications including catheter related blood stream infection and thrombosis. Particularly in the case of dialysis catheters, thrombus and sheath formation in and around the catheter may necessitate catheter removal because of its adverse impact upon flow rates and patient safety concerns. Generally, larger diameter catheters such as dialysis catheters are more likely than smaller diameter catheters to cause venous stasis and turbulent blood flow, which may be contributing factors to thrombus formation owing to characteristics such as increased catheter surface area and non-laminar blood flow. As a result, the catheter access site may become inaccessible, infected or otherwise damaged, thus leading to potentially life threatening complications if the ability to administer dialysis treatment is compromised. Moreover, any thrombus formed from catheter implantation could detach from the access site and migrate to other locations within the vascular, thus possibly causing other complications such as pulmonary embolisms. According to the Kidney Disease Outcomes Quality Initiative (“KDOQI”) guidelines, more than half of patients having long-term catheters are removed within a year due to complications. Many of these patients have end-stage renal disease and rely upon the integrity of chronic dialysis catheters as the means through which they receive dialysis treatment. Because of the inherent risks associated with such long-term placement, dialysis catheters are associated with a relatively high rate of mortality.
Current treatments for chronic dialysis catheter complications include the use of thrombolytic fluids to disrupt thrombus formation, and the administration of intravenous antibiotics to combat infection. Both of these treatment modalities are designed to treat such complications rather than to prevent them. Moreover, a fluid lock, as known in the art, is used to prevent clot formation within the catheter between dialysis sessions. Typically, a heparin-saline fluid solution is infused into the full length of the catheter lumens. Recently, the use of coatings on the outer surfaces of dialysis catheters has been proposed for the prevention of thrombus formation. Although short term results of coated catheters have demonstrated a reduction in thrombus formation relative to uncoated catheters, longer term results are not as positive. It is believed that the decreased efficacy over time is at least partially attributable to the dissipation of the coating in situ.
As an alternative to catheter coatings, the permanent binding of biologically active moieties to catheter polymer chains or polymer surfaces has been studied. In U.S. Pat. No. 6,127,507, which is incorporated herein by reference for all purposes, it is proposed to use certain fluoroalkyl surface-modifying macromolecules in admixture with elastomers for the manufacture of blood-contacting medical devices. It is believed that the use of such macromolecules can result in a reduction in thrombosis formation on the medical device surfaces. While additives such as fluoropolymers and other materials may impart beneficial properties to implantable medical devices, their addition to polymeric materials used to manufacture the medical devices may also adversely impact mechanical properties. The purity of such additives may also adversely impact these properties.
There is a need to provide dialysis catheters that are capable of preventing thrombus formation during prolonged indwelling periods, thus avoiding the need for interventional treatments such as the administration of thrombolytic fluids and antibiotics. The prevention of thrombus would also result in a decrease in infections, an increase in dialysis efficacy, and a lower incidence of access loss due to premature catheter removal. Moreover, because complications relating to vascular access are the leading cause of hospitalization for hemodialysis patients, the prevention of thrombus formation on dialysis catheters would have a significant impact upon healthcare costs.